A successor communication scheme LTE (Long Term Evolution) to W-CDMA and HSDPA has been discussed in W-CDMA standardization group 3GPP, and OFDM and SC-FDMA (Single-Carrier Frequency Division Multiple Access) are being discussed as radio access schemes for downlinks and uplinks, respectively. For example, see 3GPP TR 25.814 (V7.0.0) “Physical Layer Aspects for Evolved UTRA” (June 2006).
The OFDM is a transmission scheme where a frequency band is segmented into multiple smaller frequency bands (subcarriers) and data is transferred over the respective frequency bands. The subcarriers are densely arranged without mutual interference while they are overlapped, which can realize fast transmission and improve utilization efficiency of frequency.
The SC-FDMA is a transmission scheme where a frequency band is segmented and the segmented different frequency bands are used for multiple terminals for transmissions, resulting in reduction in interference among the terminals. In the SC-FDMA, transmit power has small variations, which can reduce power consumption of the terminals and realize broad coverage.
In the LTE, two types of CP, long CP and short CP in different lengths, are provided for the OFDM as CP (Cyclic Prefix) for reducing influence of inter-symbol interference due to delay waves. For example, the long CPs are applied in cells with large cell radii or at transmissions of MBMS (Multimedia Broadcast Multicast Service) signals while the short CPs are applied in cells with small cell radii. In the case where the long CPs are applied, six OFDM symbols are included in a single slot. In the case where the short CPs are applied, on the other hand, seven OFDM symbols are included in a single slot.
Meanwhile, a mobile station must generally detect a cell providing the mobile station with better radio quality in a radio communication system using the W-CDMA, the LTE or others based on synchronization signals or others at power up, during standby, during communications or at intermittent reception in communications. This process is referred to as cell search in a sense that a cell to be connected to in radio links is searched for. A cell search method is determined based on a time duration required for the cell search and processing load for mobile stations in the cell search. In other words, the cell search method must be performed with a less time duration required for the cell search and less processing load for mobile stations in the cell search.
In the W-CDMA, two types of synchronization signals, P-SCH (Primary SCH) and S-SCH (Secondary SCH) are used for the cell search. Also in the LTE, it is being discussed that the two types of synchronization signals P-SCH and S-SCH are used for the cell search.
For example, one cell search method where a P-SCH having one sequence and a S-SCH having multiple sequences are transmitted every 5 ms is being discussed. For example, see R1-062990 “Outcome of cell search drafting session”. In this method, downlink reception timings from cells are identified based on the P-SCH while received frame timings are detected and cell specific information such as cell IDs or cell groups (group IDs) is identified based on the S-SCH transmitted in the same slot. In general, a channel estimation value derived from the P-SCH can be used to demodulate and decode the S-SCH. For grouping of cell IDs, a cell ID for the relevant cell is then determined among cell IDs belonging to the detected cell group ID. For example, the cell ID may be derived based on a signal pattern of a pilot signal. As another example, the cell ID may be derived based on the demodulation and decoding of the P-SCH and the S-SCH. Alternatively, the cell ID may be included as an information element of the S-SCH without the grouping of cell IDs. In this case, the mobile station can detect the cell ID at the time point of the demodulation and decoding of the S-SCH.
In the case where the above-mentioned cell search method is applied, however, S-SCHs transmitted in different sequences from multiple cells may be demodulated and decoded in an inter-station synchronous system, where signals from the different cells are synchronized with each other, based on channel estimation values derived from P-SCHs transmitted in the identical sequences from the cells, which may degrade transmission characteristics for the S-SCHs. The transmission characteristics herein include a time period required for the cell search. On the other hand, in an inter-station asynchronous system where signals from different cells are not synchronized with each other, reception timings of the P-SCHs transmitted from the multiple cells are different from each other, and thus the above-mentioned problem cannot be arise.
In order to avoid the above-mentioned S-SCH characteristic degradation in the inter-station synchronous system, it is being discussed to apply a cell search method where two or more P-SCH sequences, for example, three or seven P-SCH sequences, are used. For example, see R1-062636 “Cell Search Performance in Tightly Synchronized Network for E-UTRA”. Alternatively, some methods for transmitting P-SCHs at different transmission intervals for cells have been proposed to avoid the S-SCH characteristic degradation in the inter-station synchronous system. For example, see R1-070428 “Further analysis of initial cell search for Approach 1 and 2—single cell scenario”. In this method, P-SCHs having different reception timings from multiple cells can be used for demodulating and decoding S-SCHs, which can prevent the above-mentioned S-SCH characteristic degradation.
Meanwhile, it is desirable that a larger number of P-SCH sequences as proposed in the document R1-062636 or more kinds of P-SCH transmission intervals as proposed in the document R1-070428 be used in view of cell design. If a small number of P-SCH sequences or less kinds of P-SCH transmission intervals are used, there is a higher likelihood that the P-SCHs sequences are the same in adjacent cells or there is a higher likelihood that the P-SCH transmission intervals are the same in adjacent cells, which may lead to a higher likelihood that the S-SCH characteristics may degrade in an inter-station synchronous system.
In addition, the above-mentioned time period required for the cell search, that is, the cell search transmission characteristics and processing load of mobile stations for the cell search have a trade-off relationship. Thus, it can be preferably selected through parameter configuration or operational manner which of the cell search transmission characteristics or the mobile station processing load for the cell search is considered to be more important.